A condenser microphone includes a condenser microphone unit functioning as an electro-acoustic transducer having high impedance and thus needs an impedance converter including, for example, a field-effect transistor (hereinafter referred to as “FET”). An impedance converter needs an operation power source. The voltage of the operation power source limits the maximum output level of the condenser microphone. This causes distortion of an output signal when the acoustic pressure of sound waves inputted to the condenser microphone unit is so high as to exceed the maximum output level of the unit.
Such distortion of an output signal is prevented using an attenuator called a “pad” attenuating the input level to the impedance converter. The pad includes a capacitor connected in parallel to the condenser microphone unit and attenuates the input signal level of the impedance converter in response to the ratio of capacitance of the capacitor to that of the condenser microphone unit. This can prevent an excessive input to the impedance converter.
Meanwhile, the impedance converter generates inherent noise. The noise level is constant independent of an input signal level. This decreases the signal-to-noise ratio of a condenser microphone output when the pad is used to attenuate the input level to the impedance converter for prevention of distortion due to an excessive input.
A conventional condenser microphone is known that prevents a decrease in the signal-to-noise ratio by converting an unbalanced output of the impedance converter into a balanced output to reduce the distortion of an audio signal outputted from the impedance converter (for example, see Japanese Unexamined Patent Application Publication No. 2006-101302).